Measuring system for borehole radiocativity



Nov. 8, 1949 s. KRASNOW ET AL IEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY '7 Sheets-Sheet 1 Original Filed Oct. 24, 1939 Nov. 8, 1949 f s. KRASNOW ET AL 2,487,053

MEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed Oct. 24. 1939 '7 Sheets-Sheet 2 8., 1949 s. KRASNOW ETAL IEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed Oct. 24, 1939 '7 Sheets-Sheet 3 Nov. 8, 1949 s. KRASNOW ETAL MEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed Oct. 24, 1939 7 Sheets-Sheet 4 II/fl/I/l/I/l/AI/1/11/11/1/1!!! v.3

S. KRASNOW ET AL MEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed Oct. 24, 1939 "Nov. 8, 1949 I '7 Sheets-Shoat 5 Nov. 8, 1949 s. KRASNOW ET AL 2,487,058

MEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed on. 24, 1939 7 Sheets-Sheet s I Amp.

lsc :54 3 I L Ava/M 5) Nov. 8, 1949 s. KRASNOW ET AL 2,487,058

MEASURING SYSTEM FOR BOREHOLE RADIOACTIVITY Original Filed Oct. 24, 1939 '7 Shets-Sheet v lo I' 3 '25 1:1 I66 I59 I22 I60 I61 I E I22 402 54 1&7 "I69 Fl 21 Fl .27

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iatented Nov. 8, 1949.

MEASURING. SYSTEM FOR BOREHOLE 'BADIOACTIVITY Shelley Krasnow, Arlington County,

F. Curtiss, Montgomery County, Md.,

Va., and Leon assignors,

by mesne cute, to said Krasnow Original application October 24, 1939, Serial No. 301,078. Divided and this application March 28, 1944, Serial No. 528,472

This invention relates to an improved method and apparatus for measuring radioactivity, and has particular reference to a method and apparatus for measuring radioactivity in inaccessible locations, such as in bore-holes or at considerable depths in bodies of wate One object of the invention is to provide a method and apparatus useful for locating deposits of minerals having radioactive properties. Another object of the invention is to provide an apparatus by which one may measure radioactive properties continuously from the top to the bottom 01 a bore-hole, indication, and a permanent record, of the radioactivity at various depths.

In locating deposits of radioactive minerals it is often the custom to drill a number of boreholes in localities where such deposits might exist. It is further the .practice to bring samples or cores of the drilled material to the surface of the earth, and there examine them for radioactivity by well-known methods and apparatus. This method has several drawbacks. First, a deposit of ore may exist close to the bore-hole, but not be traversed by it, by which the deposit will be missed. Second, it is possible to make an error in ascertaining the exact depth from which a core or sample has been taken. Finally, it is rarely possible to bring all of the core to the surface. a certain percentage always being lost in the drilling or handling.

It is further known that dep'oflts of petroleum are often markedly radioactive as compared with the surrounding rock material. This is believed to be due to the superior absorptive property of petroleum for radium emanation. Natural gas and ground water are also known to be somewhat more radioactive than their surrounding rock material. In drilling for either petroleum or natural gas, or ground water, it is desirable to know the exact level at which the strata having these are traversed by the drilled hole. This is often dimcult to determine, particularly when drilling has been done by the rotary" method, in which the use of mud under pressure tends to wall oi! the strata. Often too, the drilled holewill be l ned with a metallic casing, which casing by accident or intention may seal oil! strata having the desired iluid.

It is the intention in the present invention to provide an apparatus so sensitive, and a method appropriate to its use, that the relatively faint radioactivity of oil and ground water may be detected in place in a bore-hole. An apparatus sensitive enough to serve this function will by its nature diflerentiate between the different though faint radioactivities of the rock material. Rock materials, dependent upon their origin and dependent upon the minerals contained in them,

diilerent radioactivities. 'I'huait has been have and have both an immediate 3 Claims. (Cl. 250-835) found that granite, shales having organic materials embodied therein, sedimentary rocks containing zircon, and rock materials having mica associated with them, are all slightly more radioactive than for example limestone or chalk deposits. Sandstones will differ in their natural radioactivity, depending upon the minerals contaminating them. Organic deposits, such as coal, oil and natural gas, as mentioned above, petrified vegetable matter, etc., will show higher radioactivities than for instance limestone and chalk.

Thus, with an apparatus as sensitive as that described herein it will be possible to difierentiate between diiferent layers of rock by the differences in their radioactivities. Each layer in an area will have a characteristic radioactivity, just as it has a characteristic chemical composition, and for the same reason. Thus, the radioactivity of a layer will serve as a variety of marker, serving to identify the layer wherever it might be in an area.

It thus becomes possible to identify rock layers in different bore-holes drilled in an area and thus correlate the strata.

Further objects of the invention described are to obviate the difliculties mentioned and secure the advantages mentioned above.

Reference is had to the accompamring draw- 1118s in which:

Figure 1 represents a simplified form oi appa- -ratus for giving a continuous photographic rec- 0rd of radioactivity at various depths.

Figure 2 shows a cross-section of the members used in Figure 1, taken across the plane 2-2.

Figure 3 shows a vertical cross sectional view of another type of apparatus adapted to giving a photographic record of radioactivity.

Figure 4 shows a more convenient form of apparatus for measuring radioactivity at various depths in a bore-hole.

Figure 5 shows a cross sectional view of the element I I shown in Figure 4, taken across the plane 8-5.

Figure 6 shows the sensitive element employed in the apparatus shown in Figure 5.

Figure '7 shows a cross section of the element in Figure 6 taken across the plane 6-8.

Figure 8 shows the circuit diagram of the apparatus shown in Figure 4.

Figure 9 shows a vertical cross sectional view of still a further type of apparatus which may be used for the purp se described.

Figure 10 shows the circuit diagram for an improved apparatus constructed along the lines of that shown in Figure 9.

Figure 11 shows essentially the same apparatus as is shown in Figure 9 with the addition of another electroscope to serve as a control.

Figure 12 shows a means of rendering strata artificially radioactive.

tensity.

Figure 13 shows a modification of the apparatus designed to permit perforation of the easing in a well.

Figure 14 shows still another apparatus for measuring and recording radioactivity at various depths.

Figure 15 shows another type of apparatus for measuring radioactivity at various depths and giving an immediate indication at the surface of the ground of the value of the radioactive in- Figure 16 shows a detail of the lower portion of the apparatus shown in Figures 14 and 15.

Figure 17 shows a detail of the recording apparatus shown in Figure 14.

Figure 18 shows still another apparatus for measuring radioactivity, which apparatus employs radio transmission conveying information to the surface of the ground.

Figure 19 shows the electrical circuit suitable for use in the modification shown in Figure 15.

Figure 20 shows a modification of Figure 19 for radio transmission to the surface of the ground.

Figure 21 shows schematically a system for conveying information to the surface of the ground by means of mechanical waves.

Figure 22 shows a circuit for causing a change in frequency with change of radioactive intensity.

Figure 23 shows still another circuit serving the same function as that shown in Figure 22.

Figure 24 shows still another circuit, utilizing a glow discharge lamp, and serving to vary the generated frequency in relation to the change in radioactive intensity.

Figure 25 shows a modification of the circuit shown in Figure 24. I

Figure 26 shows an apparatus for measuring radioactive gradient.

Figure 27 shows a modification of the apparatus indicated in Figure 26 to render measurements of radioactivity more dependable.

Figure 28 shows an apparatus for measuring radioactive intensity with the interposition of a filter.

. Referring now particularly to Figure 1, I shows a flexible band, preferably metallic, wound upon a reel 2 operated by a crank 3. The band passes over a measuring wheel 4 and into the borehole. A photographic film is fed from a reel in operated by a crank it. to apply cement to the band I at regular intervals. This cement is fed by a brush 8 from tank 9.

In operation the band I and photographic film 8 are reeled together down into the borehole. Because of the limited strength of the photographic A lobed member I serves.

film, it is found desirable to cement the latter to the band I at intervals and thus relieve it of tensile stress. As the film I and band I are reeled 'out together, the member 1 applies spots of cement In at intervals throughout the length of the band I. This serves to cement the film and band together at intervals. The cement used may be of such nature that it will allow the ready separation of the two members when they are removed from the borehole. The photographic film reeled out is left in the borehole for a considerable period, which may be as much as several days. It is then wound upon the reel, removed and developed. If a markedly radioactive layer such as R exists, the film will be found to show a light spot. By measuring the length of fllm to this spot. the depth of the layer R may be determined. A film reinforced with strands of fiber or metal may be used and thus the necessity of using the band I obviated. It is further evident that the film may be coated with any of the standard intensifying materials commonly used in preparing medical X-ray films.

The film may also be coated with a substance to render it impervious to any harmful liquids which may exist in the borehole. An opaque coating may be applied to the film to allow its use in daylight. If the coating is properly chosen,

it will not materially hinder the passage of rays from radioactive material.

In the modification shown in vertical cross-section in Figure 3. a cartridge 51 is provided. In this is mounted a clock driven motor 58 which serves to wind-photographic film 60 upon spool 59, the film originally being on spool 6|. Cover 82 is removably fastened to container 51 by means of any suitable connection, preferably a threaded connection. A gasket 83 serves to make a fluid tight seal. The ring 84 serves for lowering and raising the apparatus in the borehole. The cartridge 51 is made of suitably heavy material to prevent collapse thereof under the high fluid pressure which may exist in a liquid-filled borehole. However, in order to reduce the absorption of radiation by the cartridge a thinned portion 85 is provided. Since this is of small area the material here may be made quite thin. In operation the clockwork 58 would be so set as to maintain the film 60 at a fixed position for a predetermined time, after which it would advance the film so as to expose a new portion thereof, and would serve to expose this new portion again for a predetermined. period. The operator knowing the length of these periods would maintain the cartridge at a fixed depth for each such period. Because of the slowness with which the film would be affected, it would not be necessary in most cases to provide any'shield to protect the film from exposure while the apparatus is being raised and lowered.

A more convenient form of the apparatus shown in Figure 4 employs a cartridge II suspended in the borehole byaconducting cable I2. The cable I2 passes over a measuring wheel I3 and thence onto a reel I4 operated bya crank I5. A pair of slip-rings Ila and Nb fastened to the shaft of the reel II have bearing upon them the brushes Itand I'I. These brushes are connected through the medium of wires I8, III, to a recording element I9. Referring now to Figures 5 and 6, the cartridge II consists essentially of a radioactivesensitive member 23 mounted at the bottom of a pressure-tight cartridge 20. A rack 2I holds the element 23 and serves further to hold batteries 24, vacuum tube 25, and relay 21. Springs 22 serve to prevent violent contact of the frame 2I with cartridge 20. A cap 3I is fastened by means of a threaded or other connection 30 onto one end of cartridge 20. A fluid tight seal is had by the use of gasket 83. The wires necessary to convey the signals from the cartridge I I pass through insulating bushing 34 and are looped onto ring 32 and thence pass to the surface. In this way the wire serves also for raising and lowering member II.

The sensitive element 23 consists essentially of a sealed glassvessel 35 which has within it a conducting ring 38 connected to wire 39 passing through seal 40. Through the axis of ring 38 there passes another conducting member 36, in the form of a wire or filament. This member 38 passes through seal AI and is further anchored against mechanical movement by being fastened to the bottom of the vessel 31. Container)! is filled with any desired gas such as air, at a amas pressure which may be as little as a few centimeters of mercury or as much as atmospheric, and is then sealed off, after which it may be used for long periods of time. without further attention. In operation the members 88 and 88 are kept at a high potential relative to each other by means of batteries 48 and 44 operating through tacts 28 and 29, thus allowing a current to flow through wires l2, slip-rings Ma and Nb, brushes l8 and I1, electro-magnet 88 and battery 49. The electro-magnet 50 serves to attract armature 8| which further serves to move pen 82 across the tape 53 kept in constant uniform motion by means of drum 54 operated by driving means 55.

The operation of the apparatus is as follows: The members 38 and 88 are charged at a controlled rate to a high potential relative to each other by means of the batteries 48 and 44 operating through leak 41. In the presence of radioactive material the gas in the container 35 will be partially ionized and will thus change the potential of the member 38s This will result in a change of potential of the grid 48 which will reduce the current normally flowing between filament 45 and plate 48 of vacuum tube 25. This will in turn reduce the current in relay 21 sufll ciently to allow its armature 28 to be retracted, closing the circuit between member 28 and contact 29. The closing of this circuit will cause a current to flow through slip-ring b, brush II, battery 49, electromagnet 58, brush l8, and slipring Ma. The energizing of electromagnet 88 will cause armature 8| to move pen 82, causing a break in the line traced on tape 88.

Upon the operation of the circuit in this fashion, the potential of member 88 will be restored to its original value increasing the fllamentetoplate current in tube 25, energizing relay 21 and thereby causing the circuit made by members 28 and 29 to open.

Upon the further ionization of the gas in container 88 the operation above described will be repeated. Thus the frequency of the pulses finale ly received by pen 52 will be a measure of the radioactivity of the material in the vicinity of member 28. It will be noted that the rays given oil by radioactive substances have considerable penetrating power and can therefore easily penetrate the shield 28 even if the latter be made of metal. To reduce the absorption of these rays by the metal, however, that portion of the cartridge 20 which houses the member 28 is provided with thinner walls than the remainder; a construction made possible by the smaller diameter of the said portion. It will be noted further that even if a metallic casing such as 88 exists in the bore-hole the presence-of a radioactive layer such as R may be noted because of the easy penetration of the rays through the thickness of metal ordinarily employed for casing.

Another type of apparatus which ma be employed is shown in vertical cross section in F 8- ure 8. In this, cartridge 88 is provided with a threaded cap 12 sealed-by gasket 14 and raised and lowered by ring l8. Within the cartridge 88 is a frame 81a which holds an ionization chamber 88 connected to an electroscope 18, which may be of any suitable type, but which is shown schematically as a gold leaf electroscope employing leaves -'|l. Springs 89 serve to cushion the shocks imparted to the frame 81a. A circumferential thin portion 81 of the cartridge is provided to reduce the absorption of the rays in passing to the chamber 88. In use the electroscope is charged to a high potential relative to the outer walls of the ionization chamber 48. It is then mounted on frame 81a, inserted in cartridge 88, the latter sealed with cap I2, and the whole lowered into the well. The apparatus is allowed to remain at a depth where presence of radioactive material is suspected, for a suitable length of time. It is then raised and the alteration in charge on the electroscope noted. A second electroscope similar to electroscope 18 may be mounted in the cartridge" and shielded from radiation, to serve as a control. Thus, the cartridge 68 may be made longer and an electroscope and ionization chamber, entirely shielded with lead, mounted above the chamber 88. This arrangement is shown in Figure 11, the control electroscope and ionization chamber being designated respectivelyas 89 and 80.

A somewhat more elaborate arrangement than is shown in Figure 9 is that shown schematically in Figure 10. In this, means are provided to charge the electroscope periodically on its charge falling of! by a definite amount. The frequency with which the electroscope is charged is a measure of the ionization current flowing and thus of the intensity of radiation in the vicinity. Specifically, the apparatus consists of an ionization chamber having an outer conducting wall I8 into which is fastened a stopcock 11 through which a suitable gas, such as air, may be passed into the chamber. An electrode 14 passes through an insulator l8 and thence into the electroscope 19. A lamp 81,'surrounded by alight tight housing 88, and having a focusing lens 88, casts a beam of light on photo-cell 82. This beam will fall on the photo-cell 82 only if the leaf H is in the discharged'position. In such event a current passes through cell 82, relay 8|, electromagnet 84, and energizing battery 88. The operation of the relay 8| closes contacts 19 and 80, thus causing battery I8 to recharge the electrode 14, and thus leaf ll. Whenever this event occurs, pen 84a is caused to move across chronograph tape 85, and thus produce a kink in the line traced by the pen. The frequency of these kinks is, therefore, a measure of the radioactivity in the vicinity of chamber 18.

In the types of apparatus shown in Figure 5 and Figure 10, the chronograph and entire recording system may be clock operated and mounted in the cartridge so that no conducting wires need pass to the surface. As a further alternative the motion of the tape may be made not a function of time but rather of the position of a measuring wheel such as l3. In the apparatus shown in Figure 10, the elements shown as 88, 84, 84a, and 85, may be mounted at the surface of the ground, similar to the mounting of element l9 in Figure 4; the rest of the apparatus being mounted in a cartridge suitable for lowering to the desired location. Thus, all of the elements shown in Figure 10, with the exception of members 88, 84, 84a, and 88, would be enclosed within a cartridge and lowered into the borehole. These last named elements would be at the surface of the ground as with similar members showninl igured.

Another apparatus which may be used for the same purpose is shown in Figure 14. This consists of a cartridge ml, which is provided with a gastight partition I and a gas-filled space 802. L0- cated preferably centrally within the space I62 is an electrode I03, carefully insulated by means of amber or other low leakage insulating material I0 3. In the partition I is mounted a valve i2i by which gas may be introduced to attain any desired pressure within the enclosure see, after which the valve I2I may be shut and the said pressure maintained. The wall IOI is made of strong material. as thin as possible to reduce the absorption of rays of radioactive material passing into the space I02. A material which will combine strength and transparency to rays from radioactive substances is utilized. Suitable materials are: magnesium alloys, aluminum alloys such as duralumin, beryllium, or beryllium alloys. A very thin steel housing may be used, the greater strength allowing the material to be so thin that absorption is not serious. The space I02 may be filled with any one of a number of gases. A suitable gas for this purpose is nitrogen, although other gases may be used with almost equally good results.

Above the partition I05 is mounted a high voltage battery I06. Connected to the positive end of battery I06 is a resistor I0I, having a high resistance. One terminal of this resistor is connected to the electrode I03, while the negative end of battery I06 is connected to the conducting casing I06.

Mounted across the ends of resistance i0! is electroscope I08, having a fixed element I00 and a movable element H0. The position of element iIIl relative to I09 will depend upon the current flowing through resistance II, which current will be a measure of the ionization existing between electrode I03 and conducting enclosure Iti. The position of element H0 is recorded photographically by a clockwork driven camera. III. The camera is shown more fully in detail in Figure 17. Here an illuminating system and photographic recording system are all mounted within "housing In. Battery I lights lamp 0, throwing light on transparent plate I I0. Plate I I8 casts light on elements I09 and H0, allowing the elements to be photographed through lens I" on film HE. A conventional clockwork motor II5 drives spool IIG taking film from spool H8, and allowingit to be reeled up on spool H2. Figure 16 shows the details of the lower portion of the apparatus shown in Figure 14, directing particular attention to the insulation employed. It is of advantage to rib or corrugate the surface of the insulation as shown, to increase the leakage path. Although element I2I is shown as a valve, in practice it may be advantageous to use a standard type of sealed-off glass Joint, as employed in the glass blowing art.

The pressure in the ionization chamber is preferably higher than atmospheric so as to give a greater ionization current, as will be familiar to those versed in the art. A pressure of several hundred pounds per square inch will be found suitable. The voltage across the chamber is made as high as possible so as to obtain an increased ionization for a given change in intensity of ionizing rays. The voltage is limited, however, by the fact that if it is made too high, ionization by collision will result and the chamber will support a steady discharge regardlessof the intensity of ionizing rays in its vicinity. The value of the resistance is such as to cause an easily measurable voltage drop across its terminals-for the usual intensity of ionizing rays. Its value will be chosen with regard to this and with regard to the requirements of the voltage indicating device. Good results may be obtained with a resistance having a value comparable and preferably approximately equal to the effective resistance of the ionization chamber. Suitable values are: a battery voltage of 130, and a resistance value of 10 megohms.

The apparatus shown in Figure 15 is the same as that shown in Figure 14, except for the means of indicating the ionization current. In this modification, the information is transmitted to the surface through wires I28, allowing immediate observation at the surface, as well as recording. The central electrode I03 of the ionization chamber is connected to element I22 which represents schematically th electrical apparatus more fully shown hereinafter. A lead I21 is connected to metallic casing IOI serving to ground certain of the elements employed in the apparatus I22. Leads I23 extend to the surface of the ground, where they may pass over a wheel such as I3. onto a reel such as It provided with slip rings such as Ida and Nb. Connecting wires such as I 6 and I8 serve to connect to frequency measuring apparatus, substituted for element I 9 shown in Figure 4.

Figure 18 shows an apparatus similar to that shown in Figure 15, the principal difference being that an element I25 is used instead of element I22, serving to generate radio signals, which signals pass to the surface of the earth where they are detected and their frequency measured. The receiving apparatus is shown schematically as I26 and will be of a type familiar to those versed in the art; This operates on a frequency measuring apparatus which may also record the frequency. If a reel such as It is used to lower the apparatus, the depth at any moment can be told and correlated with the indicated frequency. v

Referring now to Figure 19, I M and I03 represent the elements of the ionization chamber. The side MI is grounded, while the electrode I03 is connected to one terminal of a high resistance I28. The other terminal of resistance I20 is connected to the positive end of a high voltage battery I29, the negative end of this battery being grounded. A potentiometer I30 is connected across the terminals of battery I29, with its movable contact I53 connected to the cathode of a triode vacuum tube "I. A lead joins electrode I03 and the grid of a pentode vacuum tube I32. While a pentode is shown in the specific embodiment disclosed, any multi-element tube having three or more elements, and having the proper characteristics, may be used.

A conventional battery I33 is shown to provide the heater current for the tubes. A B battery I34 is shown connected to battery I33 and leading to choke I35, the other terminal of the choke being connected to the plate of tube I32. A tap is taken oil battery I30 to provide the screen grid voltage for tube I32. The plate of tube I32 is connected through condenser I36 to in-' ductance I31. Inductance I31 terminates at terminal point I38 to which is connected one I end of resistor I39. The other 'end of resistor I32 or to any other element except the plateof the same tube, with satisfactory results. it will be understood that proper biasing means will be utilised for the specific type of tube chosen.

The cathode of tube I32is connected through a conventional self-biasing arrangement I'll to the contact I53. The grid of tube I3I is connected through a conventional self-biasing arrangement Ill to inductance I31. The plate of tube m is connected to the primary of a cou- 9 transformer I42, the other end of the primary being connected to the 8" battery as shown. 'The secondary of coupling transformer "2 may be connected to leads which are brought directly to the surface of the borehole and which are connected to a suitable frequency or other measuring device, such as I3. A suitable frcquency can be chosen, high enough to be easily measurable. and low enough to avoid difficulties due to capacity and inductance effects along-the transmitting cable.

The coupling transformer may be connected to an amplifier such as I, the amplifier feeding into the external cable as shown, The amplifier will be found particularly valuable in preventing external load variations from reacting on the principal circuit and thus causing a disturbing change of frequency. Alternatively, the amplifier may feed directly into an antenna and ground arrangement, orinto what is equivalent, adipole radiating system of a type common to those versedin the radio art. This modification is shown schematically in Figure 20, I" bein the antenna or its equivalent and IE3 being the ground or its equivalent. It will be understood that for this modification, a suitable radio frequency will preferably be employed.

As as 1 further alternative, the amplifier Ill may fee into an.electromagnetic vibrator or sounding device, having an electromagnet Ill and an armature or diaphragm Ill. The alternating current output of the amplifier Ill will serve to cause an alternating magnetic field of lead from element m of the ionization chamber to the grid of tube I32. This will be a direct current amplifier, and will serve to increase the voltage change on the grid for a given change in potential on element III3. potential on element I33 is suiiicient to cause a proper voltage change of the grid, the amplifier I32 may be omitted, and a direct connection made between the lead I33 and the grid element.

The operation of the circuit may be described as follows:-. The elements I31, I33, and I40, together with tube I3I, biasing arrangement Ill, and the proper "A" and B voltage supplies, constitute an oscillatory circuit whose natural frequency is dependent on the values of inductance I31, condenser Ill, and resistance I33.

There will be an alternating voltage across the terminals of resistance I33, which voltage will be in phase with the current flowing through the resistor. This voltage will be impressed between the cathode and one grid of tube I32, and will cause in general an alternating voltage of the same frequency between the grid and plate of v the voltage in inductance I31. The magnitude of equivalent frequency in electromagnet I, which will cause the mechanical vibration of armature I33. If this armature is made to vibrate with sufficient amplitude, the mechanical vibrations caused thereby may be made strong enough to allow their transmission to the surface of the earth where they can be detected by a detector device such as a microphone I59. amplified by amplifier I60, and their frequency measured by frequency measuring device IOI. It will be understood that the constants in the circuit shown in Figure 19 will be chosen in the case aforementioned so as to give a frequency approximately within the audio-frequency range. The frequency may be made quite low, even below the audible frequency. This may be done either by selectionsof the proper constants of the circult shown in Figure 19, or by the use of a scalthis voltage will be dependent upon the amplification due to tube I32. Any out-of-phase voltage across inductance I3'I will have the effect of changing the apparent value of the inductance and will thereby cause a change in the frequency generated by the oscillatory circuit. 1

Any increase in radioactive intensity will alter the effective resistance between the electrodes IIII and I03. Through the agency of battery I 29, an increased current will flow through the circuit composed of battery I23, resistance I23, and electrodes Ifll and I03. This increased current will cause a greater voltage drop between terminals of resistance I23, which increased voltage drop, after amplification by amplifier I32, -will be impressed across the cathode and a grid of tube I32. If the screen grid voltage has been properly adjusted, any change in the potential of the grid connected to amplifier I 32, will cause a change in the effective amplification factor of tube I32. This change, as described previously will cause a change in the out-of-phase voltage impressed across inductance I31 and will thereby cause a change in the natural frequency of the oscillatory circuit described herein. The alternating current flowing through inductance I3'I will induce voltages of equal frequency in transformer I42 and consequently in amplifier I".

It is therefore seen that an alteration of radioactive intensity will cause a related and functionally connected change in frequency in the output of amplifier I33.

As in the modification shown voltage of battery I29 should be so chosen as to obtain the maximum ionizing eifect without actual breakdown, and the value of resistance I28 should be of such value as to cause a significant voltage change across the cathode and grid of tube I32. The values of the constants in the remainder of the circuit should be chosen so that with the -voltage changes normally obtained across the cathode and grid of tube I32, a sumcient change in frequency will be obtained in the output.

While a variety of vacuum tubes may be used for elements I3I and I32, a suitable set will be AnamplifyingstageI32maybeinsertedintlie I3 hadbyusinganRCAtypeitQb'ltubeaselement Where the change in,

in Figure 14,the

I3I and an RCA type #050 tube as element m. Suitable values for the other elements are as follows:

I29 l85volts I34 135 volts I28 10 megohms I10 1800 ohms and .01 mf. I38 .002 mf.

I31 700 microhenrys I40 20 mmf.

m 50,000 ohms and .0005 mi. I39 100 ohms As described herein, the frequency may vary over a wide range, depending upon the particular mode of transmission "of information to the surface. A suitable frequency is one megacycle.

Figure 22 shows still another modification making use of alternating currents only across the electrodes IN and I03 of the ionization chamber. I43 represents the inductance of the plate resonant circuit of a standard vacuum tube oscillator. The mode of construction of such an 'oscillator will be well understood by those versed in the art and no further explanation thereof need be given here. Inductance I44 constitutes an element which is inductively coupled to element I43. One terminal of inductance I44 is connected to electrode I03, while the other terminal is connected to the terminal Il of the ionization chamber. The elements of the circuit should be so proportioned that the maximum voltage developed across elements IN and I03 will be of the proper value for the particular mode of construction and particular pressure utilized in the ionization chamber. The circuit constants are further chosen so as to give the desired frequency, which may be any usual audio or radio frequency. Any change in the conductivity of the gas between elements IOI and I 03 will cause an altered current to flow in inductance I44. This will have the eflect of altering the natural frequency of the system composed of the plate resonant circuit and the inductively coupledelement I44. This frequency may be transmitted to any other inductively coupled element I45, which element will serve the function of the secondary of transformer I42.

Suitable values employed in the above modiflcation are a voltage of between I00 and I50 volts across the elements of the ionization chamber, and a natural frequency in the vicinity of two megacycles in the oscillatory circuit.

Another system which may be employed is shown in Figure 23. Here element I43 is an inductive element in the plate resonant circuit of a conventional vacuum tube oscillator. Across the terminals of this inductance are connected condenser I46 and resistance I41 in series. Across the terminals of condenser I49 are connected the elements I03 and IN. Any change in the current flowing between electrodes IN and I03 will cause an alteration in the effective natural frequency of the system composed of the elements shown. This change may be detected in an additional inductively coupled element I45, which again may be connected in place of the secondary of transformer I42.

Still another modification may be utilized for obtaining a frequency change in radioactivity in the vicinity of elements IM and I03. This is shown in Figure 24. Here high voltage battery I43, is connected with. its positive end to element I03 of the ionization chamber. Element IOI is connected to one terminal of condenser I49, the

other terminal being connected to one end of the primary I5I of a transformer. The other terminal of the primary I5I is connected to a glow discharge lamp I50, the other terminal of the discharge lamp being connected to element III.

In operation a current will flow between electrodes IOI and I 03 depending upon the ionizing effect of rays in the vicinity thereof. This current will serve to charge condenser I43 at a rate dependent upon the relation between the capacity of condenser I49 and the effective impedance of the gas between elements III and I03. As soon as the potential diiference across the terminals of condenser I49 reaches the ignition voltage of glow lamp I50, a discharge will take place in the glow lamp and the voltage across condenser I49 will drop to the extinguishing voltage of glow lamp I50. Thereupon, the charging of condenser I43 will again commence and will proceed until the voltage reaches the ignition voltage of glow lamp I50 at which time the cycle will berepeatcd as before. The oscillations in the circuit including the primary of transformer III will be transferred inductively to secondary I52 from which it can be transmitted to the surface of the ground or used in other ways to signal in the same manner as shown for the secondary of transformer I43.

A further modification making use of a glow discharge lampis shown in Figure 25. Here element I03 is connected to the positive end of a high voltage battery I29. the negative end of the battery being connected to the primary I5I of a transformer. The other end of the primary is connected to one terminal of a glow discharge lamp I50, the other terminal of the glow discharge lamp being connected to terminal Ill.

, Across the terminals IN and I03 is placed a condenser I49.

The operation of this circuit is analogous to that. of the modification shown in Figure 24, the diilerence in impedance of the ionization chamber causing a different frequency of discharge of the glow discharge lamp. The pulses thus generated are transmitted to the secondary I52 as before.

It will be understood that if a glow lamp is used as the discharge device it will flicker at a rate dependent upon the radioactive intensity in the vicinity of the ionization chamber. The rate of flicker may be observed visually if the glow lamp is at an accessible locality. The rate of flicker may also be observed indirectly by photographic means. Thus the glow lamp may be substituted for the apparatus shown as '0 in Figure 17, and may be allowed to-record on the photographic film II 3. A series of streaks will be obtained on the photographic film. the number of streaks per unit length of film being directly related to the radioactive intensity. The glow lamp lamp.

13 may be caused to act upon a photographic device or photocell as described above.

Though a glow discharge lamp has been mentioned as a proper circuit element in the modifications shown in Figures 24 and ,25, it will be appreciated that in its place may be substituted another element having non-linear negative resistance characteristics. I Figure 26 shows an apparatus which may be utilized to measure what may be termed as the radioactive gradient along the length of a borehole. This apparatus is comprised of elements such as shown in Figure 16 in duplicate. and mounted at a substantial axial distance from each other. Each unit' is connected to its associated measuring circuit I22, the outputs of the two circuits being each connected to separate frequency measuring systems at the'surface of the ground. The two frequency measuring systems may be interconnected so as to superpose one frequency on the other and give the difference of the two frequencies as a result. In this way a measure will be obtained of the relative radioactivity of the rock materials at the respective levels of each of the ionization chambers. Thus the gradient or rate of change of radioactivity may be detected.

This modification will permit further distinguishing the actual radioactivities of the strata from the possible individual erratic behavior of each of the measuring elements.

If the latter feature is sought rather than the actual measurement of gradient of radioactivity, the measuring elements may be placed close together and their combined effect noted.

This modification is shown in Figure 27.

Figure 28 shows a modificationof the apparatus shown in Figure 14, suitable for measurement of radioactive intensity through a filter. It will be understood that in certain areas little contrast will be noted in radioactive intensity throughout the length of the borehole. Advantage may be taken of the fact that different radioactive mateto radium will appear greater than that due to thorium, and the difference may be noted. The

filter may also be found particularly valuable in 4 cases where radioactivematerlal is used as an indicator, as will be hereinafter described. Different materials may be introduced in the borehole, each having different radioactive properties. They may later be identified by measurements taken with a filter. v

The apparatus shown in Figure 28resembles' that shown in Figure 14, except that an outer cylindrical shield or filter I84 has been placed completelysurrounding the cartridge llll. A latch I is held by a spring in an indentation I85 in the filter. This latch may be operated by a solenoid I61 actuated by wires I68 which pass to the surface of the borehole. 0n passing an energizing current through wires I68, solenoid I61 will cause latch I65 to be withdrawn from indentation I86, allowing filter I to drop till it strikes the circumferential stop i".

With filter I64 in the raised position, the"ap-. paratus will operate as previously described. The only filtering action will be that of the cartridge, and which is intrinsic in the material used. If it is desired to take the measurement with a filter,

. l4 the energizing current can be applied while the apparatus is in the borehole, which will cause the filter to assume an operative position, after which a further measurement can be taken. With the filter I against stop ll! all rays enterlng the ionization chamber radially will have to pass through the fil Since most of the rays enter the chamber in t way, the equivalent of a nearly complete filter will be obtained.

The filter may be made of any metal of substance having the desired absorbing properties.

Examples of suitable materials are copper, lead, aluminum, etc. It is understood that the filter may be incorporated with the cartridge MI, and

be made permanent, in which case only the filtered rays will. impinge on the instrument.

There will be a special advantage in the utilization of the filter about one of the units shown in collected will be of greater value than that obtained by a mere measurement of unfiltered radiation.

The apparatus shown in Figure 5 particularly, may be made extremely sensitive to the rays emitted by radioactive substances and so the sometimes faint radioactivity of petroleum, natural gas and ground water detected. ,As has been pointed out previously, this may be done in spite of any covering of mud or ofmetallic casing intervening between the walls of the borehole and the cartridge II. It is in fact, possible to run the cartridge ll inside of the standard drill pipe used in rotary drilling and thus make measurements with a minimum'of disturbance to drilling. Because of the limited absorptive power of the metals customarily used for drilling, it will be possible to detect radioactive rays through the thickness of metal in the drill pipe, or even through the several inch thickness of the drilling tools.

While, from what has been disclosed above, it is evident that strata may be differentiated from each oher by means of the quantitative difference in the amount of associated radioactive material, it will .be appreciated that strata need not necessarily be widely different in their associated radiothe petroleum, and will have a radioactivity markedly different from that of the petroleum itself. Thus if an apparatus as described above, were lowered past a formation, a sudden change would be observed in passing from rock to petroleum, another sudden change in passing from petroleum to water, and still another sudden change in passing' from water to rock, The layers might thus be easily identifiable despite the fact that their layer.

" layers will be identifiable by the j is radioactivity may be no greater or less than that of most of the rock lining the borehole.

In certain localities, petroleum in particular may be found to have a limited radioactivity; so

fluid, such as water, and a test made for radioactivity in the manner described previously. It will be seen that if any petroleum exists in the borehole, it will absorb radium emanation gas in greater proportion than the other strata, and will therefore exhibit a stronger radioactivity.

Figure 12 shows an arrangement for performing the above operation. Here a gas container is shown schematically as M. This contains the radium emanation gas (sometimes known as radon). A pump shown schematically as 92 serves to draw the gas from the tank 9i and pump it down through conduit 93. The gas emerges at the opening 94 of the conduit 93. A layer having superior absorptive properties for radon gas, such as an oil bearing layer, will absorb the gas more rapidly than the other layers. Such an absorbing layer is shown as R. In this instance the borehole is shown as being full of fluid through which the gas is bubbled.

ace-nose While radon gas has been mentioned as a suitable material, it will be appreciated that other substances having radioactive properties may be used instead. Such other substances may be radioabtive salts, either those having a natural radioactivity or those having an artificially excited radioactivity. It is only necessary for the purpose of the invention that the substance used be selectively absorbed by the layer of interest within the borehole. I

It will further be appreciated that in some cases the lack of absorption of the radioactive materials by a layer will serve to identify the In still other cases, the absorption, due to a layer, may be so great, that the area in the region of the layer will be denuded of radioactive material, and will appear less radioactive,

than the area in which the absorption is less. In all cases, however, the differences between differences in residual radioactivity.

It is obvious that any other means than thosev shown or described may be used to convey the frequency of the impulses produced by the apparatus in Figure 5 or in Figure 10, to the surface.

The elements employed in the member Ii may be combined with a perforating tool as ordinarily used for perforating casing in oil, gas or water wells. With this it will be possible to lower'the apparatus slowly until an indication of radioactivity is received. The apparatus may then bethese errors, since it is not dependent in any way on a measurement of depth.

A schematic showing of this appears in l isure 13. Here an apparatus capable of measuring radioactivity is shown schematicallyas 01. This is connected to a perforating element 99. This latter arrangement may be of any of the well known types. However, the type knownas a sun perforator." which perforates the casing by firing bullets through the casing, will be found particularly suitable. The assemblage made up of radioactive apparatus 01, connector 98 and perforating element 99 may be raised and low- 1 ered'by means of cable I00. This cable will serve both for raising and lowering and for operating the apparatus and making observations. .The cable passes over wheel l3, onto reel 90. Connection is made between the reel and apparatus.

95. Apparatus ll serves to make observations and to control the elements that are being raised and lowered.

In cases in which the apparatus shown in Figure 18 is used, it will be understood that measurements in a dry bore-hole will ordinarily be contemplated. In order to make this technique most effective, it will be preferable to employ a radio wave of wave length comparable with the diameter of the borehole. with such a wave length, the borehole will actas a type of tube guiding the radio waves to the top of the hole. Such a modification will operate even though the borehoile be filled, liquid.

co-pending cases, Serial No. 301,078, and Serial No. 472,894, new Patent No. 2,436,008.

The scope of the invention is defined by the. appended claims:

1. A method of geophysical prospecting that comprises lowering into a drill hole through the stem of the drill used to make the hole an instrument for continuously detecting radioactivity,

. vcontinuously determining the depth. at which said instrument is located in said drill hole, and

continuously recording on the surface the measurements made by said measuring instrument in correlation with the determinations of its. depth.

3. A method of geophysical prospecting during the drilling of a well bore that comprises moving a radioactivity detector at known depths within the drill stem and continuously during the move-- ment of said detector, recording the radioacnature of said formations.

SHELLEY KRASNOW. LEON F. CURTISS.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name v Date 1,933,063 Kolhorster Oct. 31, 1933 2,081,041 Kott May 18, 1937 2,219,273 scherbatskoy Oct. 22, 1940 2,313,310 Arnold Mar. 9, 1943 2,815,355 Scherbatskoy Mar. 30, 1943 or partially filled, with This application is a division of applicants 

